The present invention relates, in general to the continuous processing of food products such as bread by gas expansion and diffusion, and to a coextrusion process for producing a dual-textured, expanded and filled product.
The invention relates to, and is an improvement over, the apparatus and the process described and claimed in U.S. Pat. No. 5,120,559, of Syed S. H. Rizvi and Steven Mulvaney, the disclosure of which is hereby incorporated herein in its entirety.
Extrusion cooking of foods is a well known process which is practiced on large commercial scales in countries throughout the world. Extrusion cooking generally involves the mixing of food products in an extruder under conditions of high temperature, high pressure and high shear, with the cooked product being extruded through an exit die. Often, the extruded food product is expanded, or puffed, by the release of steam as the product exits the die. However, in such prior procedures the degree of puffing is dependent on a severe cooking process which increases the barrel wear in the extruder, drastically increases the starch solubility of the extrudate, and limits the use of heat sensitive or shear sensitive ingredients, such as flavors or proteins, so that such ingredients must be added to the food product after the extrusion and cooking process, as by spraying these ingredients onto the surface of the extrudate. Generally, in such prior processes, the extrudate must have an appreciable moisture content to enable the product to flow through the extruder. This means that a significant part of the throughput of the extruder is water, and even though much of the water will flash off so as to expand the product at the outlet of the die, nevertheless the high moisture content requires a drying step for the extruded product in many cases. Furthermore, since the moisture content affects both the expansion of the product and its cell structure and texture, it is difficult to specify both expansion and texture independently in such conventional processes.
In prior U.S. Pat. No. 5,120,559, an improved process for producing a food product is described. In this patent, a supercritical fluid (SCF) is introduced into a cooking extruder under conditions which are effective to produce a product having an improved texture, appearance, and flavor. In accordance with the invention, a conventional cooking extruder is modified to extend beyond the cooking and cooling sections to provide a section for injection, mixing and selective removal of a supercritical fluid. Such a fluid may be, for example, carbon dioxide (CO.sub.2) at a pressure greater than about 72.9 atmospheres (1072 psi) and at a temperature greater than about 31.degree. C. At this temperature and pressure, CO.sub.2 is at its critical point, and has properties of both a liquid and a gas. If the pressure is further increased, the CO.sub.2 density increases and has numerous desirable qualities which result in an improved extrusion process.
The first section of the extruder functions as a typical cooker, whereby a food product (e.g. cereal flour) is heated as by the injection of steam, and, if necessary, is gelatinized. If the product temperature is too high, a vent is provided on the extruder to release some steam to lower the temperature. Following the cooking section is a cooling section, which may include a cooling jacket to bring the temperature of the product down to a desired level. Thereafter, the supercritical fluid (SCF) is injected into the product stream at a selected location along the extruder. This fluid preferably is loaded with soluble materials such as flavoring, coloring, nutrients, etc. which are to be added to the product. The product may have a temperature in the range of 85.degree. to 95.degree. C., for example, in the region of injection of the supercritical fluid, and since this temperature is higher than that of the SCF, it causes a reduction in the fluid density, with the result that the solute material carried by the fluid is released into the product in the extruder. The product is then carried through a mixing section in the extruder which thoroughly incorporates the solute material into the product.
Although the supercritical fluid density is reduced upon injection into the product, the solubility of the fluid in the aqueous phase of the in-barrel product is still significant due to high pressure in the extruder, and accordingly, the supercritical fluid which remains in the product after mixing can be used to control the final product density by expansion, or puffing, of the product as it exits the extruder die. The extent of puffing and the addition of solutes are controlled by the amount and conditions of the SCF introduced into the extruder. The supercritical fluid can be partially or fully vented before the product exits the extruder so that the expansion of the product leaving the extrusion die is controlled, or if desired is not expanded at all. In addition, the temperature and pressure dependence on solubility of the SCF is utilized to obtain controlled hydrolysis of starches in the extrudate to obtain different characteristics in the end product.
The process of U.S. Pat. No. 5,120,559 has several advantages over the traditional extrusion cooking process. For example, the use of a supercritical fluid such as CO.sub.2 permits simultaneous expansion, flavoring, and reduction of viscosity. Further, extruded foams with different textural and mechanical properties can be produced by independently varying the mechanical energy input to the food product and thus varying starch degradation and or protein denaturation in the cooking zone, by venting the extruder if necessary, and by controlling the density (or the degree of expansion) of the extruded product by varying the flow of supercritical fluids into the cooled extruder product. If the product in the extruder is less than about 100.degree. C., then puffing due to the conversion of water to steam is prevented, and all expansion of the product upon extrusion will be due to the presence of the supercritical fluid.
The expansion, or puffing process, provided by supercritical fluids such as CO.sub.2 is much less explosive than the puffing that results from the conversion of water into steam. Accordingly, the supercritical fluid expansion produces a product with a smooth outer surface and uniform, regularly-shaped, internal pores.
A further advantage to the method and apparatus of the patent is that flavoring, coloring, nutrients, and the like added by way of the supercritical fluids are dissolved directly into the dough within the extruder and thus are incorporated directly into the product before it reaches the extrusion die, so that post-extrusion flavoring operations and the like can be eliminated. The incorporation of heat sensitive and/or volatile flavoring materials just before the exit die by including them in the supercritical fluid minimizes the loss of such materials due to heat or steam stripping of volatiles, as happens in the water vapor puffed products. Also, a supercritical fluid such as CO.sub.2, when used for product expansion, reduces the viscosity of the product in the extruder so that the pressure at the exit die is significantly reduced. As a result, throughput can be increased at a lower moisture content, without exceeding the motor torque or die pressure limitations of the extruder. The reduction in product viscosity can also lead to a lower energy requirement for operation of the extruder.
A supercritical fluid such as CO.sub.2 can be used to adjust the pH of the product in the extruder, and therefore products with the characteristics of acid modified starches can be produced. This process also allows control of the color and the functional and rheological characteristics of the extrudate.